The present invention relates to an apparatus for determining optical properties of materials and in particular optical surface properties of materials. The apparatus according to the invention and the method according to the invention will be described with reference to devices which determine the surface properties of paint coatings, in particular for vehicles. However, it is noted that the invention may also be used with other surfaces such as those of pieces of furniture.
From the prior art, various apparatus and also various measuring methods for determining surface properties are known. According to one of the known measurement methods, for example a certain object such as a diaphragm is projected across the surface onto the detector. The angle of detection here preferably corresponds to the angle of radiation. From the image which is preferably recorded by means of a camera, conclusions with regard to quality, e.g. the distinctness of image (dullness) of the surface concerned may be drawn.
In a further measurement method it is also possible to project a lighting diaphragm across the surface to be evaluated onto a detector and to receive the reflected radiation under a certain angle, preferably the angle of reflection. To this end, especially diaphragms are used, so that the quality of the surface may be evaluated also on the basis of the ratio between the radiated and the received intensity. Thus, according to this method a diaphragm is used on the detector side. This principle is also known as gloss measuring technology.
This last-mentioned measurement method is, however, also dependent on a curvature of the surface concerned. The first-mentioned measurement method is to a lesser degree influenced by the curvature of the surface.
The measurement methods mentioned are used to allow the optical impression as perceived by the user to be detected as objectively as possible. It is to be noted here that the human eye is able to perceive optical differences only subjectively and there is therefore a need to provide objective measurement apparatus and measurement methods. The above-mentioned second variant, however, is also dependent on the physical properties of the surface, such as in particular the index of refraction.
Surfaces having different indices of refraction will result here in different measurement results, simply because of the different indices of refraction. However, it may also be possible that two surfaces which optically convey exactly the same impression will still lead to different measurement results. The first-mentioned measurement method is—at least according to certain evaluation methods—not dependent on the index of refraction, but requires that sufficient light is reflected or scattered from the surface, so that an image may be received. This is very difficult particularly in the case of matt surfaces.